1. Field of the Invention
The present invention relates to an image forming apparatus, an electron beam apparatus, a modulation circuit, and an image-forming apparatus driving method.
2. Related Background Art
Japanese Patent Application Laid-Open No. Showa 53-105317 discloses a construction to generate luminance tonality in a display panel. Further, Japanese Patent Application Laid-Open No. Showa 54-137232 discloses a matrix display apparatus which selects an output from two clock-pulse generation means having different oscillation frequencies. Further, Japanese Patent Application Laid-Open No. Heisei 7-248748 discloses a liquid crystal display apparatus where an analog amplifier having a nonlinear characteristic is used to set a pulse width for a grayscale level. Further, Japanese Patent Application Laid-Open No. Heisei 8-160921 discloses a construction to quadruple luminance modulation by spatially and temporally changing two values of a digital signal.
Further, a flat display panel having a plurality of surface-conduction (SCE) type emission devices arranged in a matrix on a substrate is known. In this display panel, row-directional scanning is performed while sequentially selecting a row-direction wiring, and a signal corresponding to an image signal is applied to a column-direction wiring in synchronization with the row-directional scanning, whereby the respective SCE-type emission devices discharge electrons in accordance with the input image signal. The emitted electrons collide with phosphor or the like, thus causing light emission.
In this display panel, a gradation image is displayed by performing pulsewidth modulation on the input image signal in correspondence with its grayscale level and applying the pulsewidth-modulated signal to a column-direction wiring.
FIG. 7 shows the waveform of a pulsewidth modulation signal inputted into the display panel. As it is apparent from FIG. 7, the rising waveform of the signal is unsharp since the capacity of column (row) direction wiring is large and the current is limited by output impedance of a driver on the signal input side. Actually, the rise time is about 1 to 2 μsec. If the display panel is driven by this pulsewidth-modulated signal, the light-emission luminance is not linear with respect to input grayscale data, as shown in FIGS. 8A and 8B. In this case, the tonality representation is degraded.
FIGS. 8A and 8B show grayscale data (8 bits: 256 levels) determining a pulsewidth on the horizontal axis, and light emission luminance normalized in 256 levels, on the vertical axis. FIG. 8B, as an enlarged view of the graph of 8A, shows “0” to “32” luminance levels. The pulsewidth for one grayscale level is about 220 nsec, and the devices are respectively driven by a pulsewidth determined by (input grayscale level)×(220 nsec). In the display-panel driving waveform as shown in FIGS. 8A and 8B, within about 1 msec rising time, the display panel hardly emits light by the input data at “0” to “3” level, as apparent from FIG. 9B.
Further, in an image display apparatus which inputs an NTSC signal then converts it into a digital signal and displays an image on a display panel, an analog television signal is temporarily converted into a digital signal, then γ correction or the like using a look-up table is performed on the digital signal, and pulsewidth modulation, for example, is performed on the digital signal for image display.
In the look-up table, input/output data is, e.g., 8-bit data. At a low-luminance grayscale level, with respect to “00H” (“H” represents a hexadecimal number) input data, “00H” data is outputted; at an intermediate grayscale level, with respect to “AAH” input data, “55H” data is outputted; at a high-luminance grayscale level, with respect to “FFH” input data, “FFH” data is outputted. The converted result is used for display as an image signal having a linear characteristic.
In the luminance conversion processing using such a look-up table, the initially intended control on luminance signal can be performed excellently, however, in use of 8-bit input/output look-up table as shown in the conventional art, as no γ-correction value exists corresponding to the minimum or lower resolution of digital data, the conversion table is generated by rounding off required output data in accordance with necessity. Accordingly, the tonality (luminance resolution) of the displayed image is reduced, and the image quality of the displayed image is degraded. For example, in the conventional γ correction, the look-up table input/output characteristic is as follows. At a low luminance level, increment in input data by 4 results in increment in output data by 1; that is, if the input data is “4” or less, the output data is rounded to “0” or “1”. Accordingly, the tonality (luminance resolution) especially at a low luminance level is reduced and the image quality is degraded. In the above conventional art, the problem occurs in γ correction, however, a similar problem occurs in a similar construction when contrast conversion or the like is performed.